The proposed development of an aircraft designed to take off and fly in either a helicopter mode or an airplane mode has created certain problems. In one proposed form of such aircraft, a pair of powerplants driving a pair of rotor systems are mounted to wings outboard of the aircraft fuselage for pivotal motion with respect to the wings. This enables the rotor systems to rotate in a substantially horizontal plane for take-off and to rotate in a substantially vertical plane for propelling the aircraft.
When operating in a helicopter mode, the plane of the rotor system must be allowed to tilt as much as 10 degrees with respect to a vertical axis under the influence of cyclic blade pitch changes in order either to achieve horizontal flight, or to fly in the airplane mode or to maintain a static hover position in a substantial wind velocity. While many helicopters have been designed with articulated blades which flap to obtain rotor plane tilt, a more efficient design involves mounting the rotor system on a drive shaft gimbal. However, this requires the rotor system to maintain a forward tilt of up to about 10 degrees even though the drive shaft may be rotating about a vertical axis at speeds of 400 rpm. While spherical elastomeric bearings can be used as gimbals to carry thrust and radial loads, they are too soft torsionally to transmit the large torque loads required to drive the rotor system while accommodating the constant 10 degree nutating motion resulting from the misalignment of the rotor system relative to the powerplant shaft.
It is also known that in tilt rotor aircraft, gyroscopic precession forces are created as the rotating rotor systems pivot. Such forces, if transmitted to the powerplant shaft, can impose substantial stresses on the shaft as well as ancillary powerplant mounting structures. These forces may also cause the hub to which the rotor system is mounted to be misaligned with respect to the rotational axis of the powerplant drive shaft, and such misalignment must be accommodated while transferring substantial torque from the drive shaft to the rotor system, i.e. in excess of 4000 horsepower.
One proposal to solve the aforementioned misalignment problem included the use of a coupling between the powerplant shaft and rotor system hub. In such a coupling, a plurality of laminated elastomeric bearing assemblies were mounted at peripherally spaced locations in a solid plate which was connected between angularly offset spider arms secured to the shaft and hub. The thus-described structure was intended to provide sufficient flexibility as to permit the spider arms to transmit torque while rotating about misaligned axes. The use of such a coupling to solve the aforementioned rotor-mounting problem was not satisfactory because of its size, weight and grossly inadequate service life.
Another problem incident to tilt rotor aircraft such as described above is the need for torque to be transmitted from the powerplant shaft to the rotor system hub in a substantially constant velocity manner. In other words, a constant velocity condition exists between drive and driven members when each degree of angular displacement of the powerplant drive shaft induces exactly the same amount of angular displacement in the driven rotor system hub irrespective of the magnitude of misalignment between their rotational axes. Absence of a constant velocity relation between drive and driven members not only creates undesirable stresses within the coupling, but also results in undesirable vibrations in the rotor system and aircraft. These problems have long plagued mechanical link-type couplings, particularly when used to transmit torque between significantly misaligned rotational axes, such as the amount referenced above.
In the aforedescribed aircraft propulsion system, the rotors normally rotate unidirectionally. As a result, couplings are required to transmit torque primarily in only one direction of rotation. For various reasons, however, such as under conditions of autorotation created by powerplant failure, it is important for such an aircraft coupling to be able to accommodate transient reverse torque conditions, thereby imposing yet another design requirement on the capability of the coupling.
In addition to the aforementioned requirements, it is important for the rotor system coupling to be compact, lightweight and easy to maintain. Such a coupling must also have a predictable service life, operate satisfactorily without requiring lubrication, and produce visual evidence of wear long before anticipated replacement intervals. Moreover, such a coupling must be relatively simple in design, rugged, and easy to manufacture utilizing available aerospace manufacturing technologies.